This invention relates to gas turbine engines, and more particularly to an arrangement for supplying cooling air to turbine blades in a gas turbine engine having high turbine inlet gas temperatures.
Gas turbine engines typically comprise sequentially a compressor, a combustion section, and a turbine. The compressor pressurizes air in large quantities to support combustion of fuel in order to generate a hot gas stream for power generation. The combustion area is located downstream of the compressor, and jet fuel is mixed with the pressurized air in the combustion area and burned to generate a high pressure hot gas stream, which stream is then supplied to the turbine. The hot gas stream is directed by a plurality of turbine vanes onto a number of turbine blades mounted in rotating fashion on a shaft, with the hot gas stream causing the turbine to rotate at high speed, which rotation powers the compressor. The turbine goes through several stages, although the highest temperatures and hence the most hostile environment is produced where the hot gas stream enters the turbine, namely in the blades of the first turbine stage.
The turbine blades, particularly in the first stage, must therefore be fabricated of high temperature alloys in order to withstand not only the high temperatures of the hot gas stream but the substantial centrifugal forces generated by the high speed rotation of the turbine rotor. As turbine engines have been refined to become more energy efficient and deliver a higher output-to-weight ratio, while maintaining extended operating lifetimes with long periods between overhauls, it has become absolutely essential to deliver a cooling fluid to the turbine blades, particularly in the first stage. This cooling fluid, which is typically relatively cool air derived from the compressor, must be delivered through an internal passage in the rotor, which is rotating at high speed, to the turbine blades. These blades are typically provided with internal passages into which the coolant air is supplied, thereby enabling the turbine blades to survive the high temperature working environment which would otherwise destroy or critically damage them.
While arrangements for supplying cooling air from the compressor to internal passages in the turbine blades have been around for some time, an ever increasing concern has been the loss in efficiency of operation of the turbine caused by diverting the cooling fluid from the compressor to the turbine blades. While it is apparent that engine performance is reduced somewhat by the bleeding off of cooling air, maximizing the efficiency of the apparatus supplying the cooling air from the compressor to the turbine blades has been a series of responses to one type of loss rather than an effective analysis and response to the several different types of losses encountered in supplying cooling fluid to rotating turbine blades.
These losses include insertion losses and pumping losses. Insertion losses are encountered at the point at which the cooling air enters the turbine rotor, which is moving with a fairly high tangential velocity. These insertion losses require first that the cooling air be supplied to the turbine rotor at a minimal radius, thereby reducing the differential in tangential velocity of the rotor to the non-rotating air delivery system used to supply cooling air to the rotor.
Insertion losses include three critical losses. First, since most air delivery systems operate at fairly high static air pressures, losses in the seal areas between the turbine rotor and the stationary portion of the turbine have been high, reducing overall efficiency and requiring large quantities of air to be diverted from the compressor for cooling purposes. Secondly, frictional losses accompanying the injection of cooling air into the rotor reduce efficiency as well as drop air pressure significantly, further aggravating the seal problem by requiring higher delivery pressures. Thirdly, there are associated insertion losses known collectively as swirl loss, which is primarily the loss caused by the necessity for rotationally accelerating the cooling air once it is contained in the turbine rotor up to the tangential velocity of the turbine rotor. An additional smaller component of swirl loss is due to friction of the cooling air stream within the turbine rotor.
Finally, pumping losses are the losses encountered as the cooling air is supplied from the smaller radius at which it enters the turbine rotor to the larger radius at the base of the turbine blades, the point at which the cooling air is supplied to the turbine blades. The addition of pumping vanes or blades to add pressure to the cooling air to enable delivery to the turbine blades adds heat to the cooling air, as well as acting as a drag force on the rotor since work must be done to pump the cooling air to the turbine blades.
Accordingly, it can be seen that it is desirable to minimize these losses while supplying sufficient cooling air to the turbine blades through an air delivery system which performs only a minimal amount of work on the cooling air, thereby not heating and reducing the efficiency of the cooling air supplied to the turbine blades. In addition to being highly efficient, the cooling air delivery system must not reduce the structural integrity of the turbine rotor. In addition, it is desirable that a high pressure delivery system be avoided to prevent substantial air leakage at the point the air is transferrred from the stationary portions of the turbine engine to the turbine rotor.
The art in this area has concentrated for the most part on a single approach to more efficiently supply cooling air to turbine blades, namely, by imparting some degree of swirl to the cooling air before it is supplied to the turbine rotor, thereby minimizing some portion of the insertion losses. This technique to some degree will also reduce swirl loss, inasmuch as if it is performed effectively the cooling air is brought to a tangential velocity equaling the tangential velocity of the turbine rotor at the point at which the cooling air is supplied to the turbine rotor.
An early reference utilizing this approach is U.S. Pat. No. 2,910,268, to Davies et al, which is an apparatus for tapping air from a compressor section of a turbine engine and providing it to the interior portion of the shaft of a turbine rotor. While the Davies device was extremely ineffective and only marginally reduced insertion losses, succeeding references have further improved the technique of preswirling the cooling air so as to reduce some components of insertion losses and also somewhat reduce swirl loss. Such references include U.S. Pat. No. 2,988,325 to Dawson, U.S. Pat. No. 3,602,605 to Lee et al, and U.S. Pat. No. 3,936,215. These references use either stationary vanes or stationary nozzles to direct the cooling air in a rotary fashion prior to injecting the cooling air into cooling passages in the turbine rotor. By preswirling the cooling air, insertion losses are reduced somewhat. In addition, swirl losses at the point of injection are minimized, although when the cooling air travels through the internal passages in the turbine rotor, these swirl losses are generally not substantially reduced by the art.
These devices all possess significant problems in delivering the cooling air to the turbine blades, in that they require a primary design choice to be made. If cooling air is supplied at high pressure to the turbine rotor, there is a substantial leakage problem resulting in the loss of a significant percentage of the cooling air and resulting in reduced efficiency in the cooling operation. The other alternative involves supplying cooling air at a somewhat lower pressure and utilizing a pumping vane to move the air from the interior of the turbine rotor outward to the turbine blade. This technique necessarily involves performing a substantial amount of work on the cooling air, decreasing the efficiency of the cooling operation and causing drag on the turbine wheel as well as increasing the temperature of the cooling air supplied to the turbine blades. An example of such a pumping blade is shown in U.S. Pat. No. 3,602,605, to Lee et al.
It is therefore apparent that a substantial need exists for a more efficient way of supplying cooling air to turbine blades without requiring either high pressure supply and the resulting leakage of cooling air through the seals or the use of pumping vanes to supply air from the smaller radius at which the air is injected into the turbine rotor to the larger radius at the base of the turbine blades.